The development of a method for decomposing or removing nitrogen oxides or other harmful pollutants in the exhaust gas from automotive vehicles has been increasingly demanded due to environmental pollution. A three way catalytic converter, for removing carbon monoxide, hydrocarbons and nitrogen oxides concurrently, is a practical catalyst for purification of the exhaust gas. Cordierite coated with gamma-alumina or other heat resistant carrier supporting Pd, Pt, Rh or other noble metal is for general use.
However, in a lean-burn engine and a diesel engine, which are operated under the condition of high oxygen concentration the three way catalytic converter is ineffective because the noble metals are poisoned with excess oxygen. Although zeolites have been studied as the catalyst serving under the condition of high oxygen concentration, their structures easily break under the hydrothermal conditions. Oxide catalysts superior in heat resistance have also been studied, however, they are insufficient in catalytic activity for their practical use.
On the other hand, in thermal power plants or other fixed combustion equipments, nitrogen oxides in the exhaust gas containing excess oxygen are reduced by the oxide catalyst with ammonia as a reductant. Because ammonia has toxicity, a safety problem is created when this system is utilized for automotive vehicles or other moving combustion equipments or the fixed combustion equipments installed in an urban area.
Recently, a method of removing nitrogen oxides in the exhaust gas containing excess oxygen was developed. In the method, nitrogen oxides are absorbed by an absorbent in the lean-burn region or in the presence of excess oxygen, and nitrogen oxides are desorbed and reduced by means of the three way catalyst in the rich-burn region or in the vicinity of the stoichiometric air/fuel ratio.
Known absorbents of nitrogen oxides are complex oxides of the Ba--Cu--O system and the Mn--Zr--O system. However, a material having better and stable absorbability even at high temperatures is demanded.